This invention relates to improvements in methods and devices for ultrasonically welding composite structures and more particularly but not by way of limitation to interposing a flat energy concentrating member having a predetermined thickness and shape between two or more sheets of thermoplastic material prior to the application of ultrasonic energy to the sandwich of thermoplastic material and flat member to weld said thermoplastic material and flat member to weld said thermoplastic sheets together.
As further commercial and military space programs are contemplated it is becoming increasingly desirable to construct large structural arrays in various configurations. The material required for such arrays requires a combination of high stiffness/density and thermo-dimensional stability to minimize the dynamic excitation and distortion of such large arrays in orbit as they enter and leave the shadow of the earth. The properties, plus lightweight, make graphite fiber composite materials a natural choice over metallic materials.
For fabrication in space, graphite theremoplastics are of particular interest since this material can be heat laminated into continuous flat sheet stock on earth and indefinitely stored in reels or cassettes without refrigeration, which is required for uncured thermoset materials. The thermoplastic materials in the form of flat rolled strips can be compactly stored and sent into space orbit for fabrication and assembly in space. Full payloads are also achieved for transfer into orbit since the compact storage of construction materials during transfer permits adherence to transport center of gravity constraints.
Flat strips can be readily formed into structural shapes such as "U" channels, "I" beams, and variations of standard shapes. The strip material can be heat softened and then restored to its full rigidity during a cool down stage. Also, unlike the thermoset-graphite composites, structural shapes from graphite/thermoplastic can be joined to each other by the use of ultrasonic energy. Thus, undesirable in-space outgassing and the need for fasteners or materials are eliminated.
At present ultrasonic forming and welding of graphite thermoplastic materials appears to offer the most advanced and innovative approach to simple, highly efficient and reliable system for forming and assembling large space structures in orbit. Ultrasonic welding while showing the potential for the required high strength has in the past suffered from certain defects which have precluded producing welds having sufficient uniformity. Extensive experiments have been conducted to determine the effect of pressure, weld power, and time of the weld joint. It was found that typical weld nodes exhibit an interesting phenomenon. While bonds could be achieved using power settings from 350 to 1050 watts and pressures from 10 to 80 psi it was found that center of the weld area was burnt with no weld bond area and that the only area of actual weld bond was in a circle surrounding the burnt center area. This phenomenon also occurred at welding length times from 0.2 seconds to 2.0 seconds. Continued experiments revealed that weld time beyond a certain point was not contributing to weld improvement but was actually causing increased weld deterioration. This shortcoming has now been overcome by the present invention as will now be described in detail.